Three-dimensional isotropic metamaterial consisting of domain-structure

Whether an artificially designed negative-index structure could be regarded as a homogeneous medium or not rests with the ratio of its structural unit (man-made atom) over the operation wavelength. However, this definition is ambiguous, and usually the ratio is too large to rigorously meet the effective medium theory. In this paper a three-dimensional (3D) isotropic structure is presented which is obtained from a two-dimensional (2D) isotropic structure rotating on its axis for a circle, and the material is silver. Numerical studies confirm that both the 2D and 3D structures can realize a negative refractive index at microwave wavelengths. Observing the monitored surface current distributions and analogizing the molecular current and the magnetic domain, we suggest a new concept of domain-structure to explain the interior structure of this metamaterial, and finally conclude that the 3D structure is a kind of domain-structured and isotropic metamaterial.